kern/dl: Check for the SHF_INFO_LINK flag in grub_dl_relocate_symbols()

kern/dl: Check for the SHF_INFO_LINK flag in grub_dl_relocate_symbols()

The grub_dl_relocate_symbols() iterates through the sections in
an ELF looking for relocation sections. According to the spec [1]
the SHF_INFO_LINK flag should be set if the sh_info field is meant
to be a section index.

[1] https://refspecs.linuxbase.org/elf/gabi4+/ch4.sheader.html

Reported-by: B Horn <b@horn.uk>
Signed-off-by: B Horn <b@horn.uk>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>